1. Field of the Invention
This invention relates to improvements in tag transponder systems, or the like, and more particularly to improvements in systems and methods for reading identification data simultaneously transmitted from a plurality of tag transponders that each emit an rf signal when a query rf signal is received, each tag being associated with a respective item to be located, counted, identified, or inventoried.
2. Relevant Background
Keeping track of items is often desirable in many settings. For example, in merchandising environments, it is commonplace to require accurate merchandise inventories. In many instances, items need to be tracked as they are moved from one location to another. For example, in farming industries, it is often important to keep track of livestock as they are penned, or as they are trucked or moved from one location to another. In the transportation industry, railroad cars are commonly tracked as they move from one location to another. In manufacturing industries, especially with the development of "just in time" supply theories, it is becoming more and more important to track parts as they are being manufactured, in order that they can be made available, as needed.
In other industries, tracking items is also important. For example, tracking containers, and more specifically, tracking tanks of the type used to contain and transport gas, such as helium, hydrogen, or other gas, is of particular interest (although the invention may be useful in many other applications, as will become apparent.)
Toward the end of tracking items of interest, tags of the type that emit an rf signal have been proposed, and are finding increasing use. One kind of tag that is of particular interest is the type of tag which is attached or otherwise associated with an item to be tracked and which emits such rf signal in response to a "query" signal broadcast by a reader system. This type of tag is generally referred to as a "transponder", having both a receiver to receive a "query" signal from the reader, and a transmitter that transmits predefined data in response back to the reader.
Tags of this type are often of very small or compact size, powered, for example, by a capacitor that stores a sufficient amount of energy from the inquiry signal to power the response or answer signal. The benefit of this arrangement is that no separate power supply need be provided, resulting in reduced tag size. Tags of this type are generally quiescent until the "query" rf signal is received, then operate to "wake up" to produce the responding rf signal.
Often such tags are designed to transmit the rf signal modulated with a code unique to the tag or to the item with which the tag is associated. For example, a tag may transmit an rf signal that is pulse code modulated (PCMed) with data to indicate a particular tag identification number. The tag identification number can be correlated, for example, with a particular item, identified, for instance, by a serial number, a product SKU number, or the like.
Since usually a number of tags are associated with a plurality of individual items, the query signal usually is designed to operate in one of a couple of ways, depending on the particular system with which it is associated. One way, for example, is to broadcast the query signal at a unique frequency that excites only a respective one of a plurality of tags. In such instance, in order to inventory a number of individual items with which the tags are associated, a number of query frequencies need to be transmitted to wake up, in turn, each of the tags, and each response needs to be logged. This type system requires precise frequency tuning of both the transmitter and receiver, and a number of frequencies must be precisely defined over the set of frequencies occupied by the tags.
Another system uses a transmitter in which the query signal is at one frequency, but all of the tags wake up and simultaneously broadcast an answer signal. In this type system, generally the return signals from all of the simultaneously transmitted return signals are simultaneously received and added together to provide a summed signal. The summed signal is then examined to determine the relative number of tags (or relative number of corresponding items with which the tags are associated.) Generally, such systems are not thought to be useful to identify particular items, such as by SKU number, serial number, or the like.
As will be apparent, for the transponder tag system to be useful, a transmitter/receiver processing system is needed. Typically, such processing system generates and sends the "query" signal that wakes up the transponder, as described above, receives the answer signal sent by the tag, and processes the data sent in the answer. The data, typically PCM information, must be demodulated and processed, according to the type of information contained. However, in the past, such processing systems have not been able to process individual tag data other than to detect the answer signals in "gross", to indicate only the total number of tags that have responded to an inquiry signal. What is needed therefore is a facility that can be used to transmit but a single "wake up" signal to a plurality of tag transponders, and to simultaneously receive and decode individual tag data to provide information about the individual items with which the tag transponders are associated.